When an oil well is first drilled and completed, the fluids (such as crude oil) may be under natural pressure which is sufficient to produce on its own. In other words, the oil rises to the surface without any assistance.
In many oil wells, and particularly those in fields that are established and aging, natural pressure has declined to the point where the oil must be artificially lifted to the surface. Subsurface pumps are located in the well below the level of the oil. A string of sucker rods extends from the pump up to the surface to a pump jack device, or beam pump unit. A prime mover, such as a gasoline or diesel engine, or an electric motor, or a gas engine on the surface causes the pump jack to rock back and forth, thereby moving the string of sucker rods up and down inside of the well tubing.
The string of sucker rods operates the subsurface pump. A typical pump has a plunger that is reciprocated inside of a barrel by the sucker rods. The barrel has a standing one-way valve, while the plunger has a traveling one-way valve, or in some pumps the plunger has a standing one-way valve, while the barrel has a traveling one-way valve. Reciprocation charges a compression chamber between the valves with fluid and then lifts the fluid up the tubing towards the surface. The one-way valves open and close according to pressure differentials across the valves.
Pumps are generally classified as tubing pumps or insert pumps. A tubing pump includes a pump barrel which is attached to the end joint of the well tubing. The plunger is attached to the end of the rod string and inserted down the well tubing and into the barrel. Tubing pumps are generally used in wells with high fluid volumes.
An insert pump has a smaller diameter and is attached to the end of the rod string and run inside of the well tubing to the bottom. It is held in place by a hold-down device that seats into a seating nipple installed on the tubing. The hold-down device also provides a fluid seal.
The volumetric efficiency of the pump is reduced in wells that have gas. The compression chamber between the standing and traveling one-way valves fails to fill completely with liquid. Instead, the compression chamber contains undissolved gas, air or vacuum, which are collectively referred to herein as “gas”.
The gas may be undissolved from the liquid (“free gas”) or it may be dissolved in the liquid until subjected to a drop in pressure in an expanding compression chamber, wherein the gas comes out of solution.
The presence of gas in the compression chamber reduces the efficiency of the pump, wherein the lifting costs to produce the oil to the surface are increased. Gas takes the place of liquid in the compression chamber, reducing efficiency. This condition is known as “gas interference”.
The presence of too much gas in the compression chamber can completely eliminate the ability of the pump to lift fluid. This is because the gas in the compression chamber cannot be compressed into a pressure high enough to overcome the hydrostatic pressure on the traveling valve. This condition is known as “gas locked”, and is a type of gas interference.
In common field practice, a common method to break a gas lock in a conventional pump is to space the pump setting and tag the pump hard. This is done in an effort to jar the valve open so as to break a gas lock. Hitting the pump to open the valves causes damage to pump components and the rod string. Other prior art attempts to solve the gas lock problem have concentrated on the valves and the compression of a gas in the compression chamber.
Operating the pump in a gas locked condition is undesirable because energy is wasted in that the pump is reciprocated but no fluid is lifted. The pump, sucker rod string, surface pumping unit, gear boxes and beam bearings can experience mechanical damage due to the downhole pump plunger hitting the liquid-gas interface in the compression chamber on the downstroke. Loss of liquid lift leads to rapid wear on pump components, as well as stuffing box seals. This is because these components are designed to be lubricated and cooled by the well liquid.
Gas locking not only damages the pump and stuffing box, but can reduce the overall productivity of the well. Producing gas without the liquid component removes the gas from the well. The gas is needed to drive the liquid from the formation into the well bore.
Still another problem arises in the Texas Panhandle, where some oil fields have a minimum gas-to-oil ratio production requirement. In other words, both gas and oil must be produced. Many gas wells are unable to produce gas at their full potential because the downhole pumps are unable to lift the liquid oil, as the pumps are essentially gas locked.
Still another problem arises in stripper wells, which are wells that produce ten barrels or less of liquid each day. Stripper wells are low volume wells. The output from a stripper well is produced into a stock tank on the surface. Separation equipment, which separates the gas from the well, is not used because the production volume is too low to justify the expense of separation equipment. The gas is vented off of the stock tank into the atmosphere, contributing to air pollution and a waste of natural gas.
Still another problem arises in wells with little or no “rat hole”. The rat hole is the distance between the deepest oil, gas and/or water producing zones and the plugged back, or deepest, depth of the well bore. Conventional downhole pumps cannot pump these wells to their full potential due to the low working submergence of the pump in the fluid. The low submergence results in both liquid and gas being sucked into the compression chamber. If insufficient volumes of liquid are drawn in, the pump is gas locked. In low volume wells, the common practice is to shut the pump off for a period of time to allow the liquid to enter the well bore. But, in wells with little or no rat hole, shutting the pump off has no effect because the liquid level is low. Deepening the well bore is typically too expensive. These wells contain oil, but cannot be produced with prior art pumps.